[(E)]

I have played with the idea a bit as well. Without going into house rules etc. The basic idea is that castles can have unpowered wards built in, they are generally hidden beneath the structure of the castle and are only accessible from one secure room with in the castle (a strategic/plot point) which serves as a location with an "inside" and an "outside" as well as where power is funneled in. (I use a three to one power efficiency advantage to represent the various dedicated powerstones (defensive and preset use only) (and to provide a defender's advantage)).

The area is usually built to take advantage of any enchanting resources the mage in residence has at their disposal as well.

Edit
I keep meaning to write up what Symbol drawing can do in this situation but I never quite get around to it.

Another idea, if you want to apply a "force multiplier" to magic from a castle, is to have the built in wards function like the staff enchantment and remove any distance penalties from the defensive mage's casting within the confines of the castles walls.